Broadband transparency achieved with the stacked metallic multi-layers perforated with coaxial annular apertures

It is generally believed that, in the phenomena of extraordinary optical transmission, perfect transparency only occurs at a single or a multiple of discrete frequencies. This report presents for the first time that a stacked metallic multi-layered system, being perforated with coaxial annular apertures (CAAs), can be perfectly transparent in a broad frequency range. The phenomenon arises from the coupling of guided resonance modes in CAAs among different metallic layers. The transparency bandwidth is extended to about 40% of the central frequency with only 2-3 metallic layers. Measured transmission spectra in microwave regime are in good agreement with calculations which are semi-analytically resolved by modal expansion method.